Some years ago I was using a Raymarine AIS receiver (not transmitting) in a SeaTalkenvironment. If I remember correctly, it had a NMEA 0183 capability, which I certainly used to feed a backup GPS signal into the system. I do not recall the outputs. So you could perhaps use your old receiver only when the primary Raymarine fails.

The other thing to do is to turn off all alarms in the backup OCPN setup, which you will probably want to do anyway in the backup station. You will get own boat in red, and you will get used to it.

I did not meet an AIS system with the selection capabilities you propose. Technically it is all possible, and excluding one MMSI from _alerting_ (I would not exclude anything from _reporting_) is perhaps worth considering.

Personally, I believe all setups should be kept as simple as possible, and I never use automatic alerting, when underway.